1. Field of the Invention
The present invention relates to an electrically conductive inorganic coating (thin-film) and a method for producing the electrically conductive inorganic coating. Further, the present invention relates to a circuit board using the electrically conductive inorganic coating and a semiconductor apparatus using the electrically conductive inorganic coating.
2. Description of the Related Art
In recent years, various kinds of flexible devices have been receiving increasing attention. The flexible devices can be applied to various purposes, such as electronic paper (e-paper) and a flexible display. Conventionally, the flexible devices are produced, in a manner similar to production of devices that use glass substrates, mainly by vacuum deposition, such as a sputtering method and a vacuum deposition (evaporation) method, and patterning by photolithography. The conventional methods have excellent regeneration characteristics, and highly precise patterning can be performed. However, since the pattern is formed by temporarily depositing a coating onto the entire area of the substrate or the like first and by removing unneeded portion therefrom, the process is complicated and high cost.
Therefore, in recent years, a method using a direct drawing technique has been studied, as a method for producing flexible devices, to lower the cost for producing such devices. In the direct drawing technique, desirable patterns, such as circuits, are directly drawn on the substrates. Examples of direct drawing methods are printing methods, such as inkjet printing and screen printing, and a material liquid containing a composition material of the coating is applied to the substrate or the like to print. Since this method does not require any vacuum process, and patterning can be performed by direct drawing, it is possible to produce the devices easily and at low cost.
For example, production of wiring, such as circuits, and electrodes by using the direct drawing technique is studied. In this case, a paste-type inorganic-particle dispersant is used as a material liquid. In the inorganic-particle dispersant, electrically-conductive inorganic particles, such as metal, are dispersed in an organic solvent. The inorganic-particle dispersant is applied to the substrate or the like to form a desirable pattern, and fired to form micro circuits and electrodes.
However, flexible devices use resin substrates that have lower heat-resistance than the inorganic substrates, such as glass substrate. Therefore, in production of the flexible devices, it is necessary to keep the temperatures of the substrates lower than or equal to the heat-resistance temperatures of the substrates throughout the production processes. The heat-resistance temperatures of the resin substrates vary according to the types of the materials of the substrates, but they are normally in the range of 150 to 200° C. Even the heat-resistance temperature of a material, such as polyimide, that has relatively high heat-resistance is approximately 300° C.
Japanese Unexamined Patent Publication No. 2006-165467 (Patent Literature 1) discloses a method for producing an electronic device. In the method, an ultrafine-particle dispersant containing a metal element or a metal element compound that has an average particle diameter of less than 1000 nm is applied to the surface of the substrate to form a coating. Further, the coating is irradiated with a laser beam from the front side or the back side of the substrate to form a metal element layer or a metal element compound layer on the substrate. Patent Literature 1 discloses selective heating to limit heating to the coating by appropriately selecting the wavelength of the laser beam. The wavelength is selected in such a manner that the absorption rate of the laser beam at the substrate is low, thereby firing the coating without affecting the substrate (please refer to paragraph [0014] of Patent Literature 1).
According to the production method disclosed in Patent Literature 1, even when the substrate onto which the coating is deposited is the resin substrate, the ultrafine-particle dispersant of a metal element or a metal element compound can be fired while the temperature of the resin substrate is kept less than or equal the heat-resistance temperature of the resin substrate. However, in the method disclosed in Patent Literature 1, there is a risk that an organic substance, such as the organic solvent in which the ultrafine particles are dispersed and the dispersant that coats the surfaces of the ultrafine-particles, is not decomposed and remains when firing is carried out. When the coating that contains the organic substance, such as the dispersant and the organic solvent, is irradiated with a laser beam, the coating ablates (evaporates instantly), and the metal or metal compound particles are not united with each other. Therefore, it is impossible to obtain an electrically-conductive coating.
Especially, when the metal particles are dispersed in a colloid state, in which the metal particles are stably dispersed, there is a higher possibility that the dispersant remains without being decomposed. However, if a material liquid that does not have efficient dispersion stability is used, metal nanoparticles in the material liquid tend to cohere at a low temperature. Therefore, when the material liquid is applied in inkjet printing or the like, the metal nanoparticles cohere in the inkjet nozzle and form particles that have diameters on the order of greater than or equal to several tens of micrometers (μm), and clogging of the nozzle tends to occur. Further, the metal nanoparticles cohere in the vicinity of the nozzle, and that may cause a bad droplet flight condition. Further, when the particles cohere, the diameters of the cohered particles become varied, in other words, the diameters are not uniform. Therefore, particles that have various diameters are output from the nozzle, and it becomes impossible to apply the material to the substrate or the like so that a desirable pattern is formed. Hence, stable production becomes impossible.